1. Field of the Invention
The invention relates to a system and method for expanding a laser beam without expanding its spatial coherence.
2. Background Art
In lithography, or other environments (e.g., holography), expansion of an excimer laser beam or deep UV (DUV) excimer laser beam is necessary because an illumination system field is typically much bigger than the laser beam. Typically, laser beams are 10 mmxc3x9710 mm or 5 mmxc3x9720 mm, while an illumination field may be 120 mmxc3x9725 mm. Although the laser beam is described as having a rectangular or square cross-section, various cross-sections of light can be used. Generally, lithography devices use an arrangement consisting of one reflector and one partial reflector (or beam splitter) to preliminarily expand the laser beam in an optical multiplexer before expanding the preliminarily expanded beam further in other parts of the lithography tool. Unfortunately, expansion with typical optical devices (lenses, prisms) increases the spatial coherence of the laser and creates a speckle problem. Therefore, other optical devices can be used. The drawback of using the reflector/beam splitter arrangement is that it requires a complicated design of a xe2x80x9cstaircasexe2x80x9d partial reflector, which consists of patches of coatings having a stepwise change in reflectivity based on predetermined parameters. This arrangement requires an exact match of the size and position of the laser beam and the xe2x80x9cstaircasexe2x80x9d patch pattern. Also, a practical implementation of the xe2x80x9cstaircasexe2x80x9d partial reflector leads to uncoated areas between the patches and the expanded beam, which results in a xe2x80x9czebraxe2x80x9d pattern with dark areas cutting through bright areas of a beam cross section. Further, excimer lasers have a tendency to change the beam size and divergence over the time.
Therefore, a system and method for expanding an emitted light from a laser without changing spatial coherence of the light, without producing speckle patterns, and that eliminates the requirement for the xe2x80x9cstaircasexe2x80x9d partial reflectors is needed.
Embodiments of the invention provide an optical system comprising a laser source and a multiplexing device. The multiplexing device has a plurality of spatially separated beam splitters positioned parallel to and on a same side of a minor. The multiplexing device expands light emitted by laser source into plural beams having light intensity substantially equal to each other without changing spatial coherence. The optical system further comprises an illuminating optical system that focuses each of the plural beams and a projection optical system that projects an image of a mask illuminated with light output from illuminating optical system onto a substrate.
Other embodiments of the invention provide a light multiplexing device comprising a reflector and a plurality of spatially separated beam splitters positioned on a same side of and parallel to the reflector. The multiplexer expands light emitted by a laser source into a plurality of beams having light intensity substantially equal to each other without changing a spatial coherence of light emitted by laser.
Some advantages provided by the embodiments of the invention are that a laser beam is expanded without changing its spatial coherence and without producing speckle patterns through the use of uniform partial reflectors that are much easier to manufacture and produce than xe2x80x9cstaircasexe2x80x9d beam splitters. Another advantage is that it is less critical that the laser beam be accurately aligned with respect to beam splitters, which is critical in the previous devices.
Further embodiments, features, and advantages of the present inventions, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.